The relatively large seasonal effects, atmospheric signals, and numerous high-frequency components related to site effects make it difficult to observe and quantify the comparatively smaller signals caused by crustal motion (Tiampo et al., 2004). Most of the geophysical signals are non-stationary in nature, and thus Fourier-domain filtering will fail to remove them accurately. Time-frequency filters are one possible solution to this problem. Tiampo et al. (2008) employed the localized Hartley transformation on the GPS data in southern California to isolate post-seismic deformation after the 1994 Northridge earthquake, with some success that was limited by the number of GPS stations available at the time. George et al. (2011) proposed an extended S transform filtering approach to separate different signals in GPS time series. Using this filter, the authors have analysed the GIA process in eastern Canada with six years of continuous GPS data.
George, N. V., Tiampo, K. F., Sahu, S. S., Mazzotti, S., Mansinha, L., and Panda, G. (2011). Identification of Glacial Isostatic Adjustment in Eastern Canada Using S Transform Filtering of GPS Observations. Pure and Applied Geophysics, 1-11. Springer. doi:10.1007/s00024-011-0404-1.
Tiampo, K. F., Assefa, D., Fernández, J., Mansinha, L., and Rasmussen, H. (2008). Postseismic Deformation Following the 1994 Northridge Earthquake Identified Using the Localized Hartley Transform Filter. Pure and Applied Geophysics, 165(8), 1577-1602. Springer. doi:10.1007/s00024-008-0390-0.
Tiampo, K. F., Rundle, J. B., Klein, W., Ben-Zion, Y., and McGinnis, S. (2004). Using eigenpattern analysis to constrain seasonal signals in Southern California. Pure and Applied Geophysics, 161(9), 1991-2003. Springer. doi:10.1007/s00024-004-2545-y.